M104 on a 94 E320 runs slightly rough at idle when Air Pump is On
 

I've noticed that I have a slightly rough idle when the air pump is on. I can hear the air pump pretty good when it's on (EDIT added detail: if I'm near the engine with the hood open). I can feel a decent shake which almost feels like a slight mis-fire while it's running then the engine is as smooth as a top once the air pump clutch disengages.

Any ideas?

fwiw... I'm getting a pretty good rotten egg smell when I floor it. I can smell it in the car even with the winders closed.

The smog pump should not be that loud. I assume it's outside the car you can hear it? Hopefully not inside (with windows closed). I'm not sure, but IMO it sounds like it might be starting to seize internally? Keep an eye/ear on it, air pump seizing on the highway will shred the belt and leave you waiting on a tow. I'm not really sure how it could affect your idle otherwise unless it was loading the engine funny.

rotten egg smell
 

Rotten Egg Smell you need a new catalytic converter.